In the search for hydrocarbons in the earth, methods for evaluating and interpreting the structure of the earth's subsurface as well as the effects of stratigraphy, lithology, and pore fluid content on geophysical data in order to relate such effects to the occurrence or presence of hydrocarbons have been developed. Seismic reflection data is traditionally acquired and processed for the purpose of imaging acoustic boundaries and seismic reflection events in the subsurface. In the field of geophysics, numerous techniques for imparting seismic wave energy into the earth's subsurface formations, recording the returning reflected seismic wave energy and processing the recorded seismic wave energy to produce seismic signals or traces have been developed. The seismic signals or traces obtained generally contain a multiplicity of information including frequency, amplitude and phase, which can be related to geology, lithology or pore fluid content of the earth's subsurface. Such features of the seismic signals are generally referred to as instantaneous attributes. Additionally, interpretative techniques generally referred to as stratigraphic interpretative analysis have been developed for analyzing seismic data and for identifying and characterizing changes in lithology, geology and pore fluid content of the earth's subsurface from recurring patterns associated with reflection events in the seismic data. Examples of such focus are Quay et al. in U.S. Pat. No. 3,899,768 and Bodine in U.S. Pat. No. 4,779,237.
Traditionally, both two-dimensional and three-dimensional seismic data was used for interpretation, however, the use of three-dimensional seismic data continues to grow. Specifically, three-dimensional seismic data provides a more detailed structural and stratigraphic image of sub-surface reservoirs than can be obtained from two-dimensional data. As a result of the increased use of three-dimensional data, oil companies have increased hydrocarbon reserve estimates, obtained cost savings from more accurate positioning of delineation and development wells, improved reservoir characterization leading to better simulation models, and increased the ability to more accurately predict future opportunities and problems during the subsequent production of a field. In addition, as an exploration tool, three-dimensional seismic data reduces the risk in drilling in structurally complex areas and lends itself to reservoir quality prediction in undrilled areas.
The principal advantage of three-dimensional over two-dimensional seismic data is that three-dimensional methodologies provide the interpreter with the ability to view seismic data in horizontal “map” form rather than being limited to vertical cross-section views. Using traditional two-dimensional methods of viewing vertical profiles, it is often difficult to get a clear and unbiased view of faults and stratigraphic features hidden in three-dimensional data. Although faults are often readily seen on individual vertical seismic cross-sections, multiple vertical cross-sections must be examined to determine the lateral extent of faulting. In addition, stratigraphic changes are difficult to detect on vertical seismic lines because of the limited profile that they present. To avoid these issues, geoscientists have traditionally utilized two kinds of seismic map displays: amplitude time-slice and seismic horizon-slices. The amplitude time-slice is a horizontal plane, at a constant time, through the three-dimensional volume, which displays the amplitude of the seismic data at that time without reference to a stratigraphic horizon. An advantage of the amplitude time-slice is that an interpreter can view geologic features in map form without having to first pick seismic events in the data. A seismic horizon slice contains useful stratigraphic information because it displays the amplitude of the seismic data along a particular geologic horizon. However, the geoscientist must “pick” a stratigraphic surface prior to generating the seismic horizon display, which can be difficult and time-consuming. This also imposes an interpretive bias on the data set and requires that the interpreter has already defined the fault framework that intersects with the horizon under consideration. The other disadvantage of horizon slices is that the results can only be obtained on isolated surfaces in the three-dimensional volume and not on the whole volume.
As a result of increased computer speed and the ability to acquire and handle larger data sets, 3-D seismic data has become widely collected, and interpreted. In addition, researchers have devised methods to enhance the 3-D seismic data for various purposes, such as enhancement of fault appearance to facilitate the manual interpretation of geologic faults. For example, derivative difference (Luo et al., U.S. Pat. No. 5,724,309), amplitude difference (Luo et al. U.S. Pat. No. 5,831,935), and coherence technique (Bahorich et al, U.S. Pat. No. 5,563,949) have all been developed for enhancement of the appearance of geologic faults. However, each of these methodologies requires the manual interpretation of the geologic faults which is still time consuming.
What is needed is a semi-automated computer-based process that reduces the time consuming nature of fault interpretation, while simultaneously increasing the level of fault detail obtained.
Crawford et al. U.S. Pat. No. 5,987,388 describes an automated method of processing a fault enhanced 3-D seismic volume to locate and interpret faults. The method includes processing of individual lateral slices of the 3-D seismic volume wherein for each lateral slice, stripe artifacts are eliminated by adjusting pixel values to account for lines that are unduly bright or dim (and thus artifacts of processing). The linear features are enhanced by applying a modified Gumey-Vanderburg algorithm, such that the intensity value of each pixel is enhanced according to the extent to which the pixels reside in a line. Detection of lines in the enhanced lateral slice is then performed by summing pixel intensities over a window at varying directions, and associating, with a center pixel, an amplitude value corresponding to the maximum sum and a direction value associated with this sum. The amplitude and direction values are then used to trace lines in the data. The tracing of lines is performed by locating a maximum pixel and examining adjacent pixels of high amplitude in directions similar to the direction values of locally maximum amplitude values. The resulting vectors are then linked among lateral slices into surfaces that are representative of geologic faults.
Neff et al., U.S. Pat. No. 6,018,498 describes another method related to a computer implemented method and apparatus for automatically picking faults in a recorded three-dimensional seismic trace data volume. The method employs test planes, which are mathematically inserted into the seismic data volume to approximate dip and azimuth of a potential fault plane surface. A large number of data points, which are selected points on the seismic traces, are defined within the seismic volume, such that each test plane positioned in the seismic volume contains data points corresponding to at least a significant portion of a trace. The method then determines a factor for each data point which is representative of the probability that the point resides on a fault plane. This probability is based on planar discontinuity and average amplitude difference between corresponding traces in adjacent parallel test planes. The method selects locations, in an x, y grid, of a strip of locations having high probability of residing on a fault surface. The strip of the selected locations is smoothed to a line and used to construct fault lineament displays in seismic sections or time slices. The fault lineaments are stored in a computer data file, and conventional, stratigraphically enhanced, or other seismic data enhanced for seismic attributes is merged with the fault lineament files to create consolidated displays to aid interpretation of the data volume.
Accordingly, it would be desirable to have a computer implemented method for extracting faults from a 3-D seismic attribute cube, which improves seismic interpretation, by reducing interpretation cycle time and interpretation bias.